1. Technical Field
The invention relates to a capacitive load driving circuit using class-D (digital) power amplifier, an electrostatic transducer, a method of setting a circuit constant, an ultrasonic speaker, a display device, and a directional acoustic system, and more particularly, to a circuit configuration of class-D power amplifier that is suitable for driving the ultrasonic speaker for reproducing a sound having fine directivity by outputting a modulation wave, of which a carrier wave in an ultrasonic frequency band is modulated by an acoustic signal in an audible frequency band, from the electrostatic transducer serving as a capacitive load.
2. Related Art
An ultrasonic speaker may reproduce a sound having fine directivity by outputting a modulation wave of which a carrier wave in an ultrasonic frequency band is modulated by an acoustic signal in an audible frequency band.
As a transducer (transmitter) of the ultrasonic speaker, a piezoelectric transducer has been generally used. However, the piezoelectric transducer may obtain a high sound pressure by using a fine resonance characteristic of elements, but the frequency band thereof is very narrow. For this reason, the ultrasonic speaker using the piezoelectric transducer have problems in which the reproducible frequency band is narrow, and a reproduced sound quality is poor as compared to a loudspeaker. Therefore, various inventions have been studied so as to improve the above problems (for example, see JP-A-2001-86587).
Compared with this, an electrostatic transducer allowing an electrostatic force to act between an electrode of a vibrating film and a fixed electrode to generate a sound pressure by vibrating the vibrating film has been used as the transducer of the ultrasonic speaker (see an example of the electrostatic ultrasonic transducer shown in FIGS. 11A to 11C). The electrostatic transducer has a characteristic that can obtain a flat output sound pressure over a wide frequency range. For this reason, the ultrasonic speaker using the electrostatic transducer can improve the reproduced sound quality as compared to that using the piezoelectric transducer.
However, when the electrostatic transducer is driven by an analog amplifier, there have been problems as follows.
FIGS. 21A and 21B show examples of single end push pull circuit. A difference in loss between the case where a resistive load is driven and the case where capacitive load is driven by the general analog power amplifier will be described on the basis of the example. As shown in FIGS. 21A and 21B, the single end push pull circuit in which an NPN transistor Tr1 and a PNP transistor Tr2 are totem-pole-connected to up and down of an output stage (electrode amplifying stage) has been used in the general analog power amplifier. The output stage transistors are operated with class-A or class-AB (class-B). FIG. 21A shows an example for driving a load resistance RL serving as a resistive load, and FIG. 21B shows an example for driving a load capacitance (for example, electrostatic transducer) CL serving as a capacitive load.
FIGS. 22A and 22B are views showing an example of power loss generating at the output stage transistor (one side) of the analog power amplifier, and shows the relationship between collector-emitter voltage VCE and collector current IC of the upper transistor Tr1 shown in FIGS. 21A and 21B where the output stage transistor is operated with class-B.
In the case of the resistive load, since the phase of an output voltage (load voltage) is approximately equal to that of an output current (load current), the phase relationship between the collector-emitter voltage VCE and the collector current IC of the transistor is a reversed relationship as illustrated in FIG. 22A. That is, when the output current IC is maximum value, the collector-emitter voltage VCE is minimized. While, when the output current IC is minimum value, the collector-emitter voltage VCE is maximized.
Compared to this, in the case of the load capacitance CL, since the phase of the output voltage (load voltage) is out of phase with the output current (load current) by about 90 degrees, the phase relationship between the collector-emitter voltage VCE and the collector current IC is out of by about 90 degrees, as shown in FIG. 22B. At this time, when the output current IC is maximum value, since the collector-emitter voltage VCE is not minimized and has a large value, large loss WQ is generated in the transistor. Therefore, the power loss larger than that in the case of the resistive load is generated in the transistor.
As described above, in the case of driving the electrostatic transducer (capacitive load) by the general analog power amplifier, the power loss of the capacitive load is larger than that of the resistive load at the same output power. Accordingly, the power amplifier when the electrostatic transducer is driven by the analog power amplifier requires larger output than that when the resistive load is driven, thereby the device becomes larger.
Meanwhile, recently, class-D power amplifier for switching the output stage transistor by an audio power amplifier has been popularized (for example, see JP-A-2002-158550). The class-D power amplifier uses a power MOSFET having small ON resistance in an output stage element and reduces the loss in the output stage element by switching the power MOSFET. Since the loss in the output stage element of the class-D power amplifier is small as compared to the analog amplifier, the class-D power amplifier omits a radiator that is essential to the analog amplifier or may be miniaturized. Accordingly, it is possible to realize the amplifier of high output with a small size. For this reason, the class-D power amplifier is widely employed to, for example, in-car amplifier or portable terminal amplifier in which miniaturization and low loss are required, and AV amplifier having numerous output channels.
FIG. 23 is a view showing a general structure example of the class-D power amplifier. In the class-D power amplifier shown in FIG. 23, a PWM modulation circuit 42 modulates an input signal 41 into a digital signal of high frequency by using a PWM (Pulse Width Modulation) method or a PDM (Pulse Density Modulation) method. Then, class-D output stage 44 is driven by a gate driving circuit 43. The class-D output stage 44 uses a power MOSFET having a small ON resistance. The gate driving circuit 43 operates the power MOSFET in a saturated region, that is, performs a switching operation (ON/OFF operation). When the power MOSFET is OFF state, since the current hardly flows, the loss in the power MOSFET is approximately zero (0). In contrast, when the power MOSFET is ON state, the current flows toward the load, but the resistance of the power MOSFET at ON state, that is, ON resistance is very small as much as several mΩ to several tens of mΩ. Therefore, even though the high current flows, the loss in the power MOSFET may be restrained so as to be very low. For this reason, since the loss in the output stage element of the class-D power amplifier 40 is small as compared to the analog amplifier, it is possible to realize the amplifier of the high output with a small size.
As described above, since the output of the class-D output stage 44 becomes a switching waveform (modulated waveform), it is necessary to supply into the load after switching carrier component is removed by a low-pass filter. LC low-pass filter (LPF) 45 having small power loss is generally used as the low-pass filter.
However, the LC low-pass filter is connected to a proper load resistance and may obtain the effective frequency characteristic of the low-pass filter for the first time. For example, FIG. 24 shows an example of the frequency characteristic of second-order LC low-pass filter when cutoff frequency is set to about 50 kHz, but can understand that the response near the cutoff frequency is largely different according to the load resistance RL value.
As described above, if the load resistance RL value connecting to the LC low-pass filter is too low, the frequency characteristic becomes too dull, thereby gain degradation in the driving frequency band becomes large. On the contrary, if the load resistance RL value is too high, the frequency characteristic having large peak is generated. Impedance of the loudspeaker is generally 4 to 8 Ω. Therefore, when the load of 4 to 8 Ω is connected, the LC low-pass filter is designed in the general class-D power amplifier for audio so that the frequency characteristic becomes flat.
Unlike the loud speaker, since the electrostatic transducer is the same structure as a capacitor, the capacitance component is dominant as the impedance of the transducer. As an equivalent circuit, resistance component that is connected in series with the electrostatic capacitance component of the load, so called, an equivalent series resistance (ESR) is remarkably small, and the resistance component that is connected in parallel with the capacitance component of the load is remarkably large. Inductance component exists, but the value thereof is minute. Therefore, the inductance component is disregarded herein.
Accordingly, if the electrostatic transducer is connected with the output of the LC low-pass filter, it is equal to the state in which the output of the LC low-pass filter is opened. Therefore, the output of the LC low-pass filter shows the response having very sharp peak in the vicinity of a resonant frequency. For this reason, if driving the electrostatic transducer as the load by using the general class-D power amplifier for audio (if removing the switching carrier component by the LC low-pass filter of the class-D power amplifier), the large peak is generated in the frequency characteristic of the output (for example, see curve when R is 16 Ω in FIG. 24). Therefore, it may be impossible to obtain the flat frequency characteristic, and the operation may be unstable in the worst case. More particularly, when the electrostatic transducer is used as the ultrasonic speaker, if the flat frequency characteristic is not obtained, the reproduced sound quality becomes poor.
If an external resistance having a proper value is connected to the electrostatic transducer, it can obtain the flat frequency characteristic, but in exchange therefor the large power loss is generated due to the resistance. Since this causes the advantage of the electrostatic transducer in which the loss is small to be reduced, the above method is not preferable.
As described above, if driving the electrostatic transducer (capacitive load) as the load by using the general class-D power amplifier for audio, the large peak is generated in the frequency characteristic of the output. Therefore, it may be impossible to obtain the flat frequency characteristic, and the operation may be unstable in the worst case. More particularly, when the electrostatic transducer is used as the ultrasonic speaker, if the flat frequency characteristic is not obtained, the reproduced sound quality becomes poor.
If an external resistance having a proper value is connected to the electrostatic transducer, it can obtain the flat frequency characteristic, but in exchange therefor the large power loss is generated due to the resistance. Since this causes the advantage of the electrostatic transducer in which the loss is small to be reduced.